1. Field of the Invention
The present invention generally relates to an image forming apparatus of electrophotographic type or electrostatic recording type wherein a visualized image is formed on an image bearing member and the visualized image is transferred onto a recording material carried by a recording material bearing means. More particularly, the present invention relates to a color image forming apparatus wherein a recording material is electrostatically adhered to a recording material bearing means such as a recording material bearing sheet and visualized color images (toner images) formed on image bearing member(s) are transferred onto the recording material in a superimposed fashion by applying the electric field to the recording material, thereby obtaining a full-color image.
2. Related Background Art
There have been proposed various image forming apparatuses (so-colled color image forming apparatuses) wherein visualized images (toner images) of different colors are formed on image bearing member(s) and the toner images are successively transferred onto a single recording material in a superimposed fashion. Among them, color image forming apparatuses of electrophotographic type have been used most widely. Such color image forming apparatuses of electrophotographic type can generally be divided into a first group wherein toner images of different colors are successively formed on a single photosensitive drum as an image bearing member and the toner images are successively transferred onto a recording material carried and conveyed by a transfer drum as a recording material bearing means in a superimposed fashion and the transferred images are fixed together to obtain a color image, and a second group wherein a plurality (generally, four) of image forming stations are arranged side by side and toner images of different colors are formed on the respective photosensitive drums individually and the toner images are successively transferred onto a recording material carried and conveyed by a recording material bearing belt such as a dielectric belt in a superimposed fashion and the transferred images are fixed together to obtain a color image.
In such color image forming apparatuses of electrophotographic type, the photosensitive drum and the transfer drum or the recording material bearing belt are normally rotated at the same speed while they are contacted with each other. FIG. 6 is a schematic view for showing the above-mentioned first group wherein the single photosensitive drum and the transfer drum are used and for explaining a driving condition of the photosensitive drum and the transfer drum. The photosensitive drum 3 is rotated in a direction shown by the arrow at an angular velocity of .omega..sub.1. Further, the rotational force of the photosensitive drum 3 is transmitted to the transfer drum 9 via a gear 91 meshed with a gear 31 of the photosensitive drum 3 so that the transfer drum is rotated in a direction shown by the arrow at an angular velocity of .omega..sub.2. Now, it is assumed that a peripheral speed of the photosensitive drum 3 is V.sub.1, a radius of the photosensitive drum is r.sub.1, the number of teeth of the photosensitive drum gear 31 is N.sub.1, a radius of a pitch circle of this gear is D.sub.1, a peripheral speed of the transfer drum 9 is V.sub.2, a radius of the transfer drum is r.sub.2, the number of teeth of the transfer drum gear 91 is N.sub.2, and a radius of a pitch circle of this gear is D.sub.2.
Normally, in an image forming apparatus, it is so designed that the radius r.sub.1 of the photosensitive drum 3 becomes greater than the radius r.sub.2 of the transfer drum 9 by integral number times or vice versa, in consideration of the discrepancy in color during the formation of the full-color image (for example, refer to the Japanese Patent Publication No. 59-32792). That is to say, in the conventional image forming apparatuses, the following relations are established between the photosensitive drum 3 and the transfer drum 9: EQU r.sub.1 =k.multidot.r.sub.2 (k=. . . , 1/4, 1/3, 1/2, 1, 2, 3, 4, . . . ); EQU D.sub.1 =r.sub.1, D.sub.2 =r.sub.2 ; EQU .omega..sub.1 =1/k.multidot..omega..sub.2 ; EQU N.sub.1 =k.multidot.N.sub.2 ; EQU V.sub.1 =V.sub.2 .multidot.
However, in such conventional image forming apparatus, during the transferring of the toner images, particularly when the image is a line image such as a character, there arose a problem that the so-called "void" phenomenon in which a central portion of the character except for edge portions of the character cannot be transferred occurs.
Next, the occurrence of the void phenomenon will be explained with reference to FIGS. 7A to 7E.
FIG. 7A shows a portion of the line image in which the void phenomenon occurs (i.e., a central portion of the image (except for edge portions) is not transferred), and FIG. 7B shows a section of the toner line image L formed on the photosensitive drum 3 by the development. As shown in FIG. 7C, the recording material P carried by a recording material bearing sheet S of the transfer drum 9 is contacted with the toner line image L at a transfer station. In this case, due to the resilience of the recording material bearing sheet S, the resilience of the recording material P itself, an urging force of a transfer urging member (not shown) and the electrostatic attraction force between the recording material P and the photosensitive drum 3, the pressure is applied to the toner image L in a direction shown by the arrow. Consequently, as shown in FIG. 7D, since a central portion 1 of the toner image L is subjected to the partial pressure greater than those acting on both side portions (edge portions) 2, the cohesion between the toner particles is increased. Thus, when the recording material P is separated from the photosensitive drum 3, the toner existing on the surface of the photosensitive drum still remains on the drum surface due to the Fandelwarles' force. In this case, since the cohesion of the toner in the central portion 1 of the toner image L is strong, the toner of the central portion remains on the drum surface. As a result, as shown in FIG. 7E, only the edge portions 2 of the toner line image L are transferred onto the recording material P, with the result that the void phenomenon as shown in FIG. 7A occurs.
In order to avoid the void phenomenon, in an image forming apparatus wherein a conventional transfer roller is used to pinch a recording material between it and a photosensitive drum and to convey the recording material as disclosed in U.S. Pat. No. 5,159,393, it is considered that a speed of the transfer drum is differentiated from that of the photosensitive drum.
However, in an image forming method wherein toner images are successively transferred onto a recording material in a superimposed fashion, when the conveying mechanism comprising the nip between the photosensitive drum and the transfer roller is used, the moving speed of the recording material is not uniform because of the slip of the recording material in the nip. As a result, for example, due to the local excessive slip, there arises a problem regarding the synchronism between the toner images, thereby worsening the quality of the color image.